Conclusions: We found that neural response variability to positive socio-affective stimuli accurately separated patients from controls. It is likely that less signal variability highlights a deficit in effective emotion processing. We add to the growing literature on healthy individuals suggesting that task-specific brain signal variability contains useful information. The brain signal variability approach opens new avenues to evaluate and better understand brain function in common psychopathology. Keywords: Social Anxiety Disorder, BOLD fMRI, Variability F32. TMS-EEG Biomarkers for Combat-Related PTSD Wei Wu 1 , Corey Keller 1 , Parker Longwell 1 , Emmanuel Shpigel 1 , Duna Abu Amara 2 , Bryan Gonzalez 3 , Silas Mann 4 , Roland Hart 4 , Charles Marmar 2 , and Amit Etkin 1 1 Stanford University, 2 Steven and Alexandra Cohen Veter- ans Center for the Study of Posttraumatic Stress and Traumatic Brain Injury, NYU School of Medicine, 3 Stanford University School of Medicine, 4 New York University School of Medicine Background: Combat-related PTSD is experienced by men and women who have been in combat. Despite the large proportion of veterans returning from war with PTSD. Little is known regarding the underlying circuit abnormalities that are responsible for residual post-traumatic symptoms. Lack of this knowledge currently impedes the ability to develop circuit- informed novel therapeutic interventions. Methods: We performed concurrent single-pulse TMS-EEG (spTMS-EEG) to a large number of healthy controls (N¼ 80) and PTSD combat veterans (N ¼ 80) who were deployed to either Iraq, Afghanistan, or both. Sites in key cognitive and emotional networks were stimulated. The EEG data were cleaned using our in-house fully automated artifact rejection algorithm [1]. The spatio-temporal dynamics following the spTMS were then examined. The EEG responses were quan- titated using TMS-evoked potentials (TEPs) and event-related spectral perturbations (ERSPs) of the induced oscillations, in both the sensor and source spaces. Results: Following the anterior DLPFC stimulation, significant differences were observed for the early (<100 ms) and late (>100 ms) TEPs and ERSPs between the PTSD and healthy control groups (p < 0.05, unpaired t-tests, FDR corrected for multiple comparisons). These changes were identified across a number of key regions within the central executive network, salience network, and default mode network. Conclusions: These results reveal for the first time abnor- malities of casual information processing in PTSD patients across key cognitive and emotional brain networks. These biomarkers are critical in understanding the mechanisms un- derlying PTSD. Potential future avenues of this work include development and personalization of rTMS to normalize these circuit abnormalities. Supported By: Cohen Veterans Bioscience; Stanford Neuro- sciences Institute Keywords: PTSD - Posttraumatic Stress Disorder, TMS-EEG, Noninvasive Brain Stimulation, Brain Networks, Neural Oscil- lations F33. Feedback Modulated Changes in TPJ Connectivity in Subclinical Social Anxiety Khalil Thompson 1 , Kendrick King 1 , Negar Fani 2 , Eddy Nahmias 1 , Trevor Kvaran 1 , Jessica Turner 1 , and Erin Tone 1 1 Georgia State University, 2 Emory University Background: During interpersonal interactions, socially anxious individuals have a tendency to overestimate the like- lihood of social threat, such as rejection or betrayal. Abnormal processing in the temporoparietal junction (TPJ), a region implicated in social cognition, is believed to underlie the cognitive aberrancies that characterize social anxiety. The purpose of this study was to identify regions of the brain that exhibit connectivity with the TPJ during feedback appraisal during the iterated Prisoner’s Dilemma task. Methods: 31 non-psychiatric volunteers (Ages 18-28), cate- gorized as having high or low trait anxiety using Liebowitz Social Anxiety Scale-Self Report (LSAS-SR) scores, were scanned while playing the iterated Prisoner’s Dilemma task against a computerized confederate whom they were deceived to believe was a human co-player. BOLD data for all trials were examined using the Psychophysiological Interaction (PPI) an- alytic technique. Results: Although significant differences between anxiety groups were not revealed in a voxel-wise analysis, a cluster- based analysis found that the bilateral TPJ exhibited a positive trend of increased connectivity to the superior temporal gyrus (t(1,28)¼3.96, p<.05) in low compared to high anxious participants. Conclusions: Contrary to original predictions about differ- ences in effective connectivity associated with anxiety symptoms, low anxious actually exhibited a positive trend of activity in the STG. Strong interpretations cannot be made on these results given that they were extracted using a liberal cluster-based threshold of .05. Future research should be directed towards examining whether there are alternate nodes within the social cognition network which could indicate dysfunction in social anxiety based on alter- ations in effective connectivity. Supported By: NSF Keywords: Social Anxiety Disorder, Prisoner’s Dilemma, Theory of Mind, Functional Brain Imaging F34. Examining the Short-Term Anxiolytic Effect of Floatation-REST Justin Feinstein 1 , Sahib Khalsa 1 , Obada Al Zoubi 1 , Hung-wen Yeh 1 , W. Kyle Simmons 1 , Murray Stein 2 , and Martin Paulus 1 1 Laureate Institute for Brain Research, 2 University of Cali- fornia, San Diego Background: Floatation-REST (Reduced Environmental Stimulation Therapy), a novel body-based intervention which attenuates exteroceptive sensory input to the nervous system, has recently been found to reduce state anxiety across a Poster Abstracts S250 Biological Psychiatry May 1, 2018; 83:S129eS455 www.sobp.org/journal Biological Psychiatry